Recently, thin wires each having a wire diameter of 20 to 50 μm or so are popularly used as semiconductor bonding wires (hereinafter, referred to as bonding wires) for connecting an electrode on a semiconductor device and an external terminal together. A thermal compressive bonding technique with the aid of ultrasound is generally applied to bond bonding wires, and a general-purpose bonding device, and a capillary jig which allows a bonding wire to pass through the interior thereof for connection are used. A leading end of a bonding wire is heated and melted by arc heat inputting, a ball is formed by surface tension, and then the ball is compressively bonded on an electrode of a semiconductor device heated within a range from 150 to 300° C. beforehand. Thereafter, the bonding wire is directly bonded to an external lead by ultrasound compressive bonding.
Recently, technologies related to the structure, material and connection for the semiconductor mounting technologies are rapidly diversified, and for example, in a mounting structure technology, in addition to currently used QFP (Quad Flat Packaging) using a lead frame, new configurations, such as BGA (Ball Grid Array) using a substrate, a polyimide tape or the like and CSP (Chip Scale Packaging), are practically used, and a bonding wire which has improved the loop characteristic, the bonding property, the mass productivity, the usability, etc., becomes requisite. Regarding the connection technology of such a bonding wire, in order to directly bond a bonding wire at two locations, improvement of the bonding property of thin wires becomes requisite not only for ball/wedge bonding which is a currently popular bonding technique but also for wedge/wedge bonding appropriate for accomplishment of a fine diameter.
Materials of an object subjected to be bonded with a bonding wire are also diversified, and for a wiring on a silicon substrate and an electrode material, in addition to an Al alloy conventionally used, copper is now in practical use which is more appropriate for refined wiring. Moreover, an Ag plating, a Pd plating, etc., are applied on a lead frame, and a copper wiring is formed on a resin substrate, a tape, etc., and a film of a noble metal element like gold or an alloy thereof is often formed on such plating and wiring. It is requisite to improve the bonding property of a bonding wire and the reliability of a bonded part depending on the kinds of an object subjected to be bonded with the bonding wire.
From the standpoint of a requisition for a wire bonding technology, it is important to form a ball with a good sphericity having a clean surface at the time of ball formation, and to accomplish a sufficient bonding strength at a bonded part between the ball and an electrode. Moreover, in order to cope with lowering of a bonding temperature and thinning of a bonding wire, a bonding strength and a tensile strength at a portion where a bonding wire is subjected to wedge bonding to a wiring part on a circuit wiring substrate are also important factors.
As wire characteristics that satisfy the foregoing requisitions, it is desirable to comprehensively satisfy various characteristics such that a loop control is facilitated in a bonding process, a joining property to an electrode and to a lead is improved, any excessive wire deformation in a resin encapsulation process after bonding is suppressed, and a long-term reliability of a bonded part and a stability of the bonded part under a severe circumstance are assured.
So far, 4N-group having a high purity (purity>99.99 mass %) is mainly used as a material of bonding wires. In order to improve strength and a bonding property, a tiny amount of alloy elements are prepared. Recently, a gold alloy wire with a purity of 2N (purity>99%) having a an additive element concentration increased up to less than or equal to 1% has been put into practical use in order to improve the reliability of a bonded part. The strength can be improved and the reliability can be controlled by adjusting the kind and the concentration of an alloy element added to gold. Conversely, alloying may cause deterioration of a bonding property and increase of an electrical resistance, so that it is difficult to comprehensively satisfy various characteristics requisites for the bonding wires.
Moreover, because gold is expensive, other kinds of metal which have a low material cost are desired, and bonding wires made of copper which have a low material cost and which have a good electrical conductivity are created. According to the copper bonding wires, however, a bonding strength is deteriorated due to oxidization of a wire surface, and a wire surface is likely to be corroded when encapsulated in a resin. These are the reasons why practical usage of the copper bonding wires is not accelerated. The silver bonding wires have such advantages that silver is inexpensive than gold, has the highest electrical conductivity, and enables ball formation in an atmosphere, but has such disadvantages that the bonding reliability at a high temperature is slightly poor in comparison with Au, and is likely to be effected by sulfuration of a wire surface.
All bonding wires in practical use so far have a monolayer structure. Even if materials, such as gold and copper, are changed, alloy elements are uniformly contained in a bonding wire, and all wires employ a wire monolayer structure as viewed from a cross section of the bonding wires. A thin native oxide film, an organic film, etc., for protecting a surface may be formed on the wire surface of a bonding wire in some cases, but these are restricted in an extremely-thin region up to several atomic layer level) in an outermost surface.
In order to meet various needs requisites for the bonding wires, a bonding wire with a multilayer structure in which a wire surface is coated with another metal is proposed.
Bonding wires made of copper which has inexpensive material cost, has a good electrical conductivity, and ensures ball bonding and wedge bonding, etc., are created, and for example, patent literature 1 is disclosed. According to copper bonding wires, however, a bonding strength is deteriorated due to oxidization of a wire surface, and a wire surface is likely to be corroded when encapsulated in a resin. These are the reasons why practical usage of the copper bonding wires is not accelerated.
In the case of copper bonding wires, when the leading end of a wire is melted to form a ball, bonding is carried out while a gas is sprayed to the leading end of the wire in order to suppress any oxidization. Recently, a nitrogen gas containing 5 vol % of hydrogen is popularly used as an atmosphere gas at the time of ball formation of copper bonding wires. Patent literature 2 discloses that a copper wire is connected under 5 vol % H2+N2 when connected to a copper or copper-alloy lead frame. Moreover, it is reported in non-patent literature 1 that a 5 vol % H2+N2 gas is more desirable than an N2 gas because such a gas can suppress any oxidization of a ball surface at the time of ball formation in the case of copper bonding wires. Nowadays, a 5 vol % H2+N2 gas is standardized as a gas used when copper bonding wires are used.
As a technique of suppressing any oxidization of a surface of a copper bonding wire, patent literature 1 discloses a bonding wire in which copper is coated with a noble metal or a corrosion-resistant metal, such as gold, silver, platinum, palladium, nickel, cobalt, chrome, and titanium. Moreover, from the standpoint of a ball formability and suppression of any deterioration of a plating solution, patent literature 3 discloses a bonding wire so structured as to have a core member mainly composed of copper, a dissimilar metal layer formed on the core member and made of a metal other than copper, and a coating layer thrilled on the dissimilar metal layer and made of an oxidization-resistant metal having a higher melting point than that of copper. Patent literature 4 discloses a bonding wire comprising a core member mainly composed of copper, and an outer skin layer which contains a metal, having either one of or both of a constituent and a texture different from the core member, and copper, and which is a thin film having a thickness of 0.001 to 0.02 μm.
Various gold bonding wires with a multilayer structure have been also proposed. For example, patent literature 5 discloses a bonding wire comprising a core wire formed of highly pure Au or an Au alloy, and a coating material coating the outer circumference surface of the core wire and formed of a highly pure Pd or a Pd alloy. Patent literature 6 discloses a bonding wire comprising a core wire formed of highly pure Au or an Au alloy, and a coating material coating the outer circumference surface of the core wire and formed of highly pure Pt or a Pt alloy. Patent literature 7 discloses a bonding wire comprising a core wire formed of highly pure Au or an Au alloy, and a coating material coating the outer circumference surface of the core wire and formed of highly pure Ag or an Ag alloy.
It is desirable to cope with the most-advanced fine diameter technique and the high-density packaging technique like three-dimensional wiring by comprehensively satisfying such characteristics that loop control is stable in a bonding process, a bonding property is improved, any deformation of a bonding wire at the time of resin encapsulation is suppressed, and a long-term reliability of a bonded part is accomplished as wire characteristics of a bonding wire of mass-production.
Regarding ball bonding, it is important to form a ball with a good sphericity at the time of ball formation, and to obtain a sufficient bonding strength at a bonded part between the ball and an electrode. Moreover, in order to cope with lowering of a bonding temperature and thinning of a bonding wire, a bonding strength, a tensile strength, etc., are requisites at a portion where a bonding wire is wedge bonded to a wiring part on a circuit wiring substrate.
There is no report so far that such bonding wires with a multilayer structure are subjected to mass-production and popularly used, and the manufacturing technology thereof has not been established yet in the industries. Material designing and development of manufacturing technology are desired in order to improve the mass-production yield, the productivity, the quality stability, etc., of bonding wires with a multilayer structure.